The Boeing B-47 Bomber
The Boeing B-47 jet bomber was a major postwar innovation in combat jet design, and led to the development of modern jetliners. While it never saw major combat use, it was a mainstay of US strategic defense in the 1950s. This document provides a description and history of the B-47.
The B-47 was refined over its history, with improved bomber versions featuring more powerful engines and improved avionics, as well as a series of photographic and signals intelligence platforms. This chapter discusses the main B-47 variants. Special variants and modifications are discussed in the next chapter.
The Boeing B-47 - Origins
The B-47 arose from a 1943 US Army Air Forces (USAAF) requirement for a jet bomber / reconnaissance aircraft, which evolved into a formal request the next year. The request specified a speed of 805 KPH (500 MPH) or more; a range of 5,635 kilometers (3,500 miles); a service ceiling of 12.2 kilometers (40,000 feet); and use of the General Electric TG-180 turbojet engine, then in development.
North American, Convair, and Boeing submitted proposals. The first Boeing proposal, the "Model 424", was submitted in January 1944. It was a jet implementation of a conventional straight-wing propeller-driven bomber design, basically a scaled-down version of the Boeing B-29 Superfortress, powered by four TG-180 turbojets fitted in a pod of two engines on each wing. The US National Advisory Committee for Aeronautics (NACA, the ancestor of the modern US National Aviation & Space Administration or NASA) performed wind tunnel tests on a model of the design, or more specifically a composite of the designs submitted by the manufacturers, since the three submissions were generally similar.
By this time, the war in Europe was obviously winding to a close. General "Hap" Arnold, head of the USAAF, asked the prestigious expatriate Hungarian aerodynamicist Theodore von Karman, of the California Institute of Technology, to form up a committee of American scientists to go to Europe and examine captured German technology. The result was the "Scientific Advisory Group". One of the members was Boeing's chief aerodynamicist, George Schairer. During his visit to Germany, Schairer examined data obtained by German aircraft manufacturers on the advantages of swept wings, and became so convinced of the merits of such a design that in May 1945 he wrote a letter to Boeing management suggesting the matter be investigated.
Meanwhile, the USAAF had awarded study contracts to all three aircraft manufacturers working on the jet bomber project, as well as to the Martin company, which had also decided to join the competition.
The NACA wind tunnel tests showed that Model 424 suffered from excessive drag. Boeing engineers then went to a revised design in April 1944, the "Model 432", that retained the straight wings but had the four engines buried in the forward fuselage. However, although the Model 432 had some structural advantages, changing the engine layout didn't really reduce drag all that much. The Boeing engineers turned to the swept-wing data obtained from the Germans and promoted by Schairer. A little design work by Boeing aerodynamicist Vic Ganzer led to an optimum sweepback of 35 degrees.
Boeing then modified the Model 432 design with a swept wings and tail. resulting in the "Model 448". The Model 448 still had the four TG-180s in the forward fuselage as had the Model 432, plus two TG-180s buried in the tail. The Boeing project manager, George Martin, had decided that the company's entry into the bomber competition needed greater range and performance, and that led to six engines instead of four. Boeing submitted the Model 448 to the USAAF in October 1945, only to have it rejected immediately. The Air Force strongly disliked fitting the engines in the fuselage, since that made engine fire or disintegration catastrophic. The engines would have to be moved back out on the wings.
That led straight back to the drag problem, but the engineering team came up with a clean, elegant solution, with the engines in streamlined pods attached under the wings. This innovation led to the next iteration, the "Model 450", which featured two TG-180s in a single pod mounted on a pylon about a third of the way outboard on each wing, plus another engine slung from the wingtip.
The Air Force liked the new configuration, and so the Boeing team continued to refine it. One problem was landing gear. There was no space for landing gear in the thin wings, and trying to put conventional tricycle landing gear in the fuselage would have ruined the aircraft's streamlining and degraded its performance. Furthermore, the USAAF was now also insisting that the bomber be able to carry an atomic bomb. Since such weapons were very big at the time that meant a long bombbay, further limiting space for landing gear.
The solution was a "bicycle" landing gear configuration, with the two main gear assemblies arranged in a tandem instead of side-by-side configuration. Outrigger landing gear was to be fitted to the inboard engine pods. The concept had already been tested on a modified Martin B-26 Marauder aircraft, the "Middle River Stump Jumper", named after the Martin plant at Middle River, Maryland.
However, bicycle landing gear made it difficult for a pilot to "rotate" an aircraft into a nose-up position for takeoff. Again, the solution was simple: the landing gear was designed so that the nose-up position was the default. This little change would have a very pleasing effect on an aircraft that was already shaping up to be very elegant, giving the machine the appearance of being ready to leap into the air even when it was sitting still.
There were some other tweaks to the design, such as a wingtip extension to improve range. This had the effect of moving the outboard engines from a wingtip position to an underwing position towards the end of the wings.
The USAAF was very pleased with the refined Model 450 design, and in April 1946 the service ordered two prototypes, to be designated "XB-47". Assembly began in June 1946. People involved with the project were very excited, since they believed, correctly as it turned out, they were working on a breakthrough in aircraft design. However, there was a widespread disinterest in the machine through the rest of the Boeing company, it seems partly because it was so futuristic, leading many to dismiss it as a whizzy experimental aircraft that would be impractical for operational use. Pictures of the initial rollout of the first XB-47 prototype show only about a hundred people watching. The aircraft was given the name "Stratojet", but nobody ever really called it that in practice. In fact, the bomber would never receive any nickname that stuck through its entire history.
The XB-47 prototype first flew on 17 December 1947, the 44th anniversary of the first flight of the Wright Brothers, with test pilots Robert Robbins and Scott Osler at the controls. The aircraft flew from Boeing Field in Seattle to the Moses Lake Airfield in central Washington state, in a flight that lasted 52 minutes. There were no major problems, except that Robbins had to pull up the flaps with the emergency hydraulic system and the engine fire warning lights kept popping on, the sensor technology being very unreliable at the time. Robbins reported that the flight characteristics of the aircraft were good.
The Boeing XB-47 and B-47A
The XB-47 was an attractive, "Buck Rogers" aircraft. The 35-degree swept wings were shoulder-mounted, with the twin inboard turbojet engines mounted on a pylon in a very neat pod, and the outboard engine tacked under the wing short of the wingtip.
The airfoil was 11 times as wide as it was thick. This unusual thinness was required to attain high speed, but the wing's flexibility was a concern. It could flex as much as 1.5 meters (5 feet) up or down, and major effort was expended to ensure that flight control could be maintained as the wing flexed. As it turned out, most of the worries proved unfounded. The wings were fitted with a set of flaps that extended well behind the wing, known as "Fowler flaps", to assist in takeoffs.
The bicycle landing gear dictated by the thin wing consisted of a pair of large wheels fore and aft of the bombbay, with small outrigger wheels carried on the inboard twin-jet pods.
The performance of the Model 450 design was projected to be so good that the bomber would be as fast as fighters then on the drawing board, and so the only defensive armament was to be a tail turret with two 12.7 millimeter (0.50 caliber) Browning machine guns, which would in principle be directed by a radar-directed automatic fire-control system. The two XB-47s were not fitted with the tail turret since they were engineering and flight test aircraft, and in fact the prototypes weren't fitted with any combat gear at all.
Fuel capacity was an enormous 64,400 liters (17,000 US gallons), compared to 19,000 liters (5,000 US gallons) on the B-29. That meant that maintaining fuel trim to ensure a stable center of gravity in flight would be very critical.
The first prototypes was fitted with GE "J35" turbojets, the production version of the TG-180, with 17.66 kN (1,800 kgp / 3,970 lbf) thrust each. Early jet engines did not develop good thrust at low speeds, so to assist in takeoffs in heavily loaded condition, the XB-47 prototype was to have 18 solid-fuel "jet-assisted take off (JATO)" rockets with 4.41 kN (450 kgp / 1,000 lbf) thrust each. Nine such units were built into each side of the lower rear fuselage, arranged in three rows of three bottles.
Similarly, the great weight of the aircraft and the lack of thrust reversers made for high landing speed. Late in flight testing, following a suggestion by Air Force test pilot Major Guy Townsend, a 9.75 meter (32 foot) drag chute was introduced to provide landing deceleration. A conventional parachute would be torn to shreds, so a ribbon parachute, derived from German designs, was used instead.
A related problem was that the aircraft's engines would have to be throttled down on landing approach. Since it could take as long as 20 seconds to throttle them back up to full power, the big bomber could not do a "touch and go" and circle around for another landing attempt if something went wrong. As a result, a second, half-diameter chute was added that would be deployed on approach so that the engines could be kept throttled up until the pilot felt sure his approach was safe.
The XB-47 was designed to carry a crew of three in a pressurized forward compartment: a pilot and copilot in a long fighter-style bubble canopy, and a navigator in a compartment in the nose. The copilot doubled as tail gunner, and the navigator as bombardier. The bubble canopy could pitch up and slide backward, but since the cockpit was high off the ground, crew entrance was through a door and ladder on the underside of the nose.
Total bombload capacity was to be 4.5 tonnes (10,000 pounds). Production aircraft were to be equipped with, by the standards of the time, advanced electronics for navigation, bombing, countermeasures, and turret fire control.
The second XB-47 prototype first took the air on 21 July 1948, and was equipped with much more powerful GE J47-GE-3 turbojets with 23.54 kN (2,400 kgp / 5,200 lbf) thrust each. The J47 or "TG-190" was a redesigned version of the J35 / TG-180. The first XB-47 prototype was later retrofitted with these engines.
Flight testing of the prototypes was particularly careful and methodical, since the design was so new in many ways. The prototypes initially suffered from "Dutch roll", an instability that caused the aircraft to weave in widening "S" turns. This problem was fixed by the addition of a "yaw damper" control system that applied rudder automatically to damp out the weaving. The prototypes also had a tendency to pitch up, and this problem was solved by tacking small vanes called "vortex generators" onto the wings. The vortex generators set up turbulence to prevent separation of airflow from the wings.
Boeing test pilot Rob Robbins had originally been skeptical about the XB-47, saying that before the initial flight he had "prayed to God to please help me" through the flight. The aircraft was so unusual that he simply didn't know if it would fly. Robbins soon realized that he had an extraordinary aircraft.
In early 1948, the Air Force (now the "USAF", having become a separate service in 1947) sent up a chase plane from Muroc (now Edwards) Air Force Base in California to help calibrate the bomber's airspeed system. Robbins reported later:
"[The chase plane] was a P-80 [Lockheed Shooting Star] and Chuck Yeager was flying it. Chuck's a hell of a good pilot, but he had a little bit of contempt for bombers and a little disdain for civilian test pilots. Well, we took off, climbed out, and got up somewhere within four or five points of full throttle speed. At that point, Chuck called me on the radio and said: "Bob, would you do a 180?" I thought: Hey, Chuck's smart, he just wants to stay reasonably close to Moses Lake, he doesn't have as much fuel as I do. Well, I turned around, got stabilized, and looked for Chuck. He wasn't there. Finally, I got on the radio and said: "Chuck, where are you?" He called back and rather sheepishly said: "I can't keep up with you, Bob." So Chuck Yeager had to admit to a civilian test pilot flying a bomber that he couldn't keep up! That was something!"
By mid-1948, the Air Force's bomber competition had already been through one iteration, pitting the North American "XB-45" against the Convair "XB-46". The North American design won that round of the competition, and as an interim measure the USAF had decided to put the North American bomber into production on a limited basis as the "B-45 Tornado". The expectation was that B-45 production would be terminated if either of the remaining two designs in the competition, the Boeing XB-47 and the Martin "XB-48", proved superior.
The XB-47 was clearly an aircraft of enormous potential, but it was still so exotic that many USAF generals didn't take it seriously. At the end of July, USAF General K.B. Wolfe, in charge of bomber production, visited Boeing in Seattle, and Boeing president Bill Allen suggested that the general take a ride on the XB-47. Wolfe was reluctant, but Allen and others managed to talk him into it, and Guy Townsend gave Wolfe an amazing ride. In early August, Wolfe contacted Boeing and indicated that the Air Force wanted to place an order for ten more of the new Boeing jet bombers. A formal contract was signed on 3 September 1948.
These ten aircraft were "B-47As". They were strictly evaluation aircraft. The first was delivered in December 1950. The configuration of the B-47As was close to that of the initial XB-47 prototypes. They were fitted with J47-GE-11 turbojets, providing the same 23.54 kN (2,400 kgp / 5,200 lbf) thrust as the earlier J47-GE-3, and they also featured the built-in JATO bottles.
Four of the B-47As were fitted with the K-2 bombing and navigation system (BNS), with an HD-21D autopilot, an analog computer, AN/APS-23 radar, and a Y-4 or Y-4A bombsight. Two B-47As were fitted with the tail turret, one of them using an Emerson A-2 fire control system (FCS), another early version of the GE A-5 FCS. The eight other B-47As had no defensive armament.
The B-47As were fitted with ejection seats. The pilot and copilot ejected upward, while the navigator ejected downward. Minimum safe ejection altitude was about 150 meters (500 feet).
Although the XB-47s had been built by Boeing at their Seattle, Washington, plant, the B-47As and all following Boeing B-47 production were built at a government-owned factory in Wichita, Kansas, where the company had built B-29s in the past. The switch was made since the Seattle plant was burdened with KC-97 production and other urgent tasks, and the Air Force also thought Seattle was too vulnerable to a Soviet bomber attack.
Most of the B-47As had been phased out of service by early 1952, though one did perform flight tests for NACA for a few more years. While the Air Force put the B-47As through their paces, the Cold War was rising to full force, with a hot war flaming away in Korea. The USAF's Strategic Air Command (SAC) needed an effective nuclear deterrent to keep the Soviet Union in line; the B-47 was an excellent tool for the task, and Boeing was already working on production bombers.